The invention relates to a process for verifying and adjusting the precise value of the energy, generally being less than 2 eV, of the ions emitted by an ion source and entering a reaction chamber as a beam, which cause ion molecule reactions in the reaction chamber, the reaction products of which being detectable mass spectroscopically.
On studying ion molecule reactions, ions (e.g. Kr.sup.+, He.sup.+) of defined energy are directed to neutral molecules. The interaction of ions and molecules can then consist in a mere charge exchange. But it is also possible that it comes to a chemical change of the hit molecule, e.g. by dissociation. Mass and charge of the reaction products can be analysed in a mass spectrometer. This is done today primarily to determine the cross sections of different ion molecule reactions at different energies.
The application of the process described in the analysis of the composition of the gas bombarded with ions, especially to the measurement of air pollutants, is only possible under very certain conditions. To be able to test the exhaust gases of combustion processes for CO, NO and NO.sub.2 with a single ion beam, the use of an ion source is important, that emits ions with an energy of less than 2 eV, which on the one hand ionize the relevant contaminations, but on the other hand do not dissociate the components of the gas composition. It has to be taken care that the ion molecule reaction takes place in the vacuum under single collision conditions. Hence, the generated product ions should, in their turn, not produce further products in a manner difficult to control.
Also in those cases, where the ion molecule reactions of the kind described are only used to measure air pollutants and details of the reaction are furthermore not interesting, it is nevertheless necessary, to hold the energy of the ions impinging on the gas to be tested at a predetermined value, as the production rate of the product ions is differently energy dependent. A direct measurement of the voltage between the emission filament of the ion source and the reaction chamber can therefore only be used for a coarse adjusting of the ion energy, as already small changes of the surface coatings (as they occur especially in the presence of different gas contaminations) in the aperture systems in the ion source or also in the reaction chamber lead to changes of the contact potentials in the order of 0.1 Volts, whereby the energy of the ions is changed by just the same amount of 0.1 eV.
The process often used with scientific investigations of molecule reactions to unify the energy of the ions used for the investigation, where they are led through a buffer gas, is not possible here, where an ion beam should cause reactions in a considerably evacuated reaction chamber. The application of electrical and magnetical fields already proposed for such cases to mask out a part of the ion beam depending on its energy is too expensive for the commercial use of ion molecule reactions.
The invention is based on the fundamental idea to use just the energy dependence of ion molecule reactions, which is the reason why the stabilisation of the ion energy is regarded as being so important, for the stabilisation. To put this idea into practice quantities had to be found that indeed depend on the energy of the injected ions but not on the absolute value of the product ion current. This absolute value namely depends on different quantities, e.g., on the geometry of the arrangement, on the intensity of the ion current, on the concentration of the gas to be analysed etc., and therefore does not allow simple conclusions on the ion energy.